This invention relates to a throttle valve assembly for controlling the output power of an internal-combustion engine and, more particularly, to such a throttle valve assembly of the type which includes a throttle body having an intake passage therein, a throttle shaft journalled on said throttle body across said intake passage, a throttle lever secured to said throttle shaft, a throttle valve mounted on said throttle shaft for regulating the flow rate of the intake air flowing through said intake passage, and a second, intermediate lever pivoted on said throttle body and adapted to be moved via a throttle linkage by an accelerator, one of said throttle lever and second lever having an arcuated cam slot engaging a roller mounted rotatably on the other lever for interconnecting said levers with each other, and wherein the cam profile of said cam slot is arranged so that when the accelerator pedal is stepped on, the angular position of the throttle valve varies in response to the travel of the accelerator according to a predetermined functional relationship.
A throttle valve assembly of the type described is known in the prior art. This throttle valve assembly enables the throttle valve to move according to a preselected functional relationship with respect to the travel of the accelerator of the vehicle and thus ensures the acceleration of the vehicle in accordance with the driver's wishes. This type of throttle valve assembly is, thus, advantageous over other conventional throttle valve assemblies in which the travel of the accelerator is transferred through a throttle linkage to a throttle lever/shaft assembly without the interposition of an intermediate lever and, therefore, the angular displacement of the throttle valve is proportional to the travel of the accelerator.
In the throttle valve assembly of the type described, however, it has been customary to make the width of the cam slot, formed in one of two levers, larger than the outer diameter of the assocaited roller, in consideration of the dimensional variations that result during machining of the cam slot. As a result, a crosswise clearance is formed between the cam slot and the roller in a direction perpendicular to the axis of the cam slot. Also, for similar reasons in respect to the manufacturing technique, the length of the cam slot as measured along the axis thereof is made greater than the maximum travel of the roller which takes place as the throttle valve undergoes angular displacement from the closed or idle position to a wide-open position. As a result, a considerable lengthwise clearance is also formed between the roller and the proximal end of the cam slot when the throttle valve is in the idle position. These crosswise and lengthwise clearances result in an excessive play in the accelerator, which, in turn, causes the driver discomfort.
Further, in the prior throttle valve assembly of the type described, both the throttle lever and the intermediate lever are provided with a return spring so that the throttle shaft, and hence the throttle valve, is returned to the idle position by the sum of the spring forces of these two springs. With this arrangement, when the accelerator is released so as to decelerate the vehicle, both the throttle lever and the intermediate lever effect a return swing, due to the action of the two springs, until the return swing motion of the intermediate lever is completed. After termination of the return swing of the intermediate lever, however, the throttle lever must be moved back to the idle or rest position through the above-mentioned lengthwise clearance by only the single return spring associated therewith. This causes the return swing of the throttle valve through said lengthwise clearance to be performed inadequately and gives rise to an unnecessary play in the throttle linkage or the accelerator.
Still further, the prior throttle valve assembly uses a roller which is in the form of a hollow cylinder and is made from a single mass of material, the roller being mounted rotatably on a roller shaft. With such a roller, the friction existing between the outer surface of the roller and the cam slot differs from that existing between the inner surface of the roller and the roller shaft, so that when the friction at the inner surface is greater than the friction at the outer surface, smooth rotation of the roller is hindered, thereby increasing wearing of the cam slot. Moreover, as the outer surface of the roller is cylindrical, the outer surface comes into contact edgeways with the edge of the cam slot when the axis of the roller is inclined with respect to the lever which is provided with the cam slot, thereby resulting in abnormal wearing of the cam slot.
Another disadvantage of the prior throttle valve assembly is that an excessive force is applied to the roller/cam slot mechanism when the accelerator is pressed all the way down, such excessive force being liable to damage the mechanism.